Abstract/Summary

Late Permian (Tatarian) fluvial sediments accumulated during the final phase of orogenesis in the southern Uralian Foreland Basin (Russia). Four facies associations have been recognised which, in ascending stratigraphic order, are mudflat, sandy distributary, small gravelly-channel and large gravelly-channel. Fluvial processes were dominant, with the size of channels and grain size increasing upsection. Sediment provenance and palaeocurrents indicate an intraorogen source basin with transport to the west, across the basin axis. The highly gradational contacts between the mudflat, sandy distributary and small gravelly-channel associations suggest that these facies were part of a small (50–100 km long) prograding terminal fan characterised by downslope decreases in channel size caused by evaporation and infiltration. The overlying large gravelly-channel association is so out of proportion in terms of required discharge that it cannot be related to this small fan. It was deposited as part of a much larger terminal fan (900 km long) and represents an abrupt phase of drainage net enlargement at the Permo–Triassic boundary. A decline in thrust-related tectonic subsidence probably accounts for the overall development of the coarsening-upward succession. However, the abrupt emplacement of thick conglomerates at the top of this succession probably resulted from a change toward a more arid climate, with higher sediment yield and greater peak discharges in a drainage basin with reduced vegetation cover.